Method of making sulfur tolerant composite cermet electrodes for solid oxide electrochemical cells

ABSTRACT

An electrochemical apparatus is made containing an exterior electorde bonded to the exterior of a tubular, solid, oxygen ion conducting electrolyte where the electrolyte is also in contact with an interior electrode, said exterior electrode comprising particles of an electronic conductor contacting the electrolyte, where a ceramic metal oxide coating partially surrounds the particles and is bonded to the electrolyte, and where a coating of an ionic-electronic conductive material is attached to the ceramic metal oxide coating and to the exposed portions of the particles.

GOVERNMENT CONTRACT CLAUSE

The Government of the United States of America has rights in thisinvention pursuant to Contract No. DE-AC-0280-ET-17089, awarded by theUnited States Department of Energy.

This is a division of application Ser. No. 867,860, filed May 28, 1986,and issued Oct. 27, 1987, U.S. Pat. No. 4,702,971.

BACKGROUND OF THE INVENTION

The use of nickel-zirconia cermet anodes for solid oxide electrolytefuel cells is well known in the art, and taught, for example, by A. O.Isenberg in U.S. Pat. No. 4,490,444. This fuel electrode or anode mustbe compatible in chemical, electrical, and physical-mechanicalcharacteristics such as thermal expansion, to the solid oxideelectrolyte to which it is attached. A. O. Isenberg in U.S. Pat. No.4,597,170 solved bonding and thermal expansion properties between theanode and solid oxide electrolyte, by use of a skeletal embeddinggrowth, of for example, primarily ionic conducting zirconia doped withminor amounts of yttria, covering lower portions a porous nickel powderlayer comprising the porous cermet anode.

This anchoring of the anode nickel particles to the solid oxideelectrolyte was accomplished by a modified chemical vapor depositionprocess, usually providing a dense deposit. While this process providedwell bonded anodes, having good mechanical strength and thermalexpansion compatibility, gas diffusion overvoltages were observed duringoperation, lowering overall cell performance. Additionally, these anodeswere not found to be particularly tolerant of sulfur contaminants.

In order to reduce gas diffusion overvoltages A. O. Isenberg et al., inU.S. Pat. No. 4,582,766, taught oxidizing the nickel particles in thecermet electrode to form a metal oxide layer between the metal particlesand the electrolyte, while additionally providing porosity in theembedding skeletal member, and then reducing the metal oxide layer toform a porous metal layer between the metal electrode particles and theelectrolyte; all allowing greater electrochemical activity. Suchstructures were still not found to be particularly sulfur tolerant,however, and provide a limited number of electrochemical sites. What isneeded is a sulfur tolerant anode structure having low diffusionovervoltages coupled with long periods of acceptable performance.

SUMMARY OF THE INVENTION

The above problems have been solved and the above needs met by providingan electrode, bonded to a solid oxygen conducting electrolyte,containing particles of an electronic conductor partly embedded in askeletal member of a ceramic metal oxide, where the surface of theparticles and skeleton are contacted, preferably covered completely,with a separate, porous, gas permeable oxygen-ionic-electronic conductormaterial coating. This coating is sinter, or diffusion attached,electronically conductive, and can contain, preferably, doped or undopedceria, doped or undoped urane, i.e., uranium oxide, or their mixtures.Ceria based outer coatings are preferably doped with zirconia, thoria orlanthanide oxides. Uranium oxide based coatings are also doped withthese oxides. These oxides are known to be compatible with yttriastabilized zirconia electrolyte, and interdiffusion, if present at hightemperatures, is not detrimental to cell performance or life.

Ceria based and uranium oxide based oxides, besides being oxygen ionicconductors, exhibit considerable electronic conduction as compared tozirconia, especially at oxygen activities at which fuel cell metalelectrodes are used. This face increases considerably the active surfacearea of the electrode for electrochemical redox reactions. Adsorption ofsulfur species on active sites, which is the reason for low sulfurtolerance of the cermet electrodes, is therefore greatly reduced inseverity during electrode operation. After applying the oxide coating, athermal treatment, at from about 500° C. to about 1400° C. in anatmosphere that prevents cermet oxidation, advantageously allowselements from the ceria based or uranium oxide based oxide outer coatingto diffuse into the skeleton structure, and introduce increasedelectronic conduction to the skeleton.

The resulting coated electrodes have increased numbers of activeelectrode sites, are mechanically strong, provide low diffusionovervoltage, long periods of outstanding performance, and, veryimportantly, are more tolerant to fuel contaminants such as sulfur andother sulfur species.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be made tothe preferred embodiments exemplary of the invention shown in theaccompanying drawings, in which:

FIG. 1 is an isometric view in section of one embodiment of a tubularsolid oxide fuel cell according to this invention;

FIG. 2 is a schematic end view in section showing a fuel electrodehaving metal particles partly embedded in an oxide skeleton structure,all disposed on top of an electrolyte;

FIG. 3 is a schematic end view in section of the electrode of thisinvention having an ionic electronic conductor outer coating over boththe metal particles and the partly embedding skeleton structure;

FIG. 4 is a graph of fuel cell voltage vs. average current density,showing improvement of fuel cells after anode impregnation withlanthanum-doped ceria, as described in the Example;

FIG. 5 is a graph of electrolysis cell overvoltage vs. average currentdensity, showing overvoltage reduction of electrolysis cells after anodeimpregnation with lanthanum-doped ceria, as described in the Example;and

FIG. 6 is a graph of fuel cell voltage vs. time, showing vastly improvedsulfur tolerance of lanthanum-doped ceria electrodes, as described inthe Example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the Drawings, FIG. 1 shows air or oxygen, A, flowingthrough the center 1 of the tubular fuel cell 2. The air (oxygen)permeates through porous support tube 3 to air electrode 4 where oxygenis converted to oxygen ions. The oxygen ions are conducted throughelectrolyte 5 to fuel electrode anode 6, where they react with fuel, F,such as H₂, CO, CH₄, etc., to generate electricity. As can be seen, thefuel electrode in this configuration is an exterior electrode, where theelectrolyte 5 is in tubular form and in contact with an interiorelectrode 4.

Also shown in FIG. 1 are longitudinal space 7 containing aninterconnection 8 for making electrical connections from the underlyingair electrode to the fuel electrode 6 of an adjacent cell tube (notshown) and an electronically insulating gap 10. A metal or fuelelectrode type of material 9 is coated over interconnection 8. Adetailed description of the general operation of the solid oxide fuelcell, along with appropriate description of useful support, airelectrode, and interconnection materials, can be found in U.S. Pat. No.4,490,444, assigned to the assignee of this invention.

FIG. 2 is a much enlarged and detailed schematic illustration of thefuel electrode structure 6, where only a skeleton embedding structure isapplied to metallic particles comprising the anode. In FIG. 2, anelectrolyte 5 is contacted with particles 11 of a metallic conductorwhich forms the fuel electrode. A ceramic skeletal coating 12 coverssmall portions of the particles 11 and binds them to the electrolyte 5.

The cell and electrolyte 5 can have a variety of shapes but thepreferred shape is a tubular, as that configuration is the most usefulfor solid oxide electrochemical cells. The electrolyte material 5 istypically an oxide having a fluorite structure or a mixed oxide in theperovskite family, but other simple oxides, mixed oxides, or mixtures ofsimple and mixed oxides can be used. The preferred electrolyte materialis stabilized zirconia, a readily available commercial material. Thezirconia may be stabilized, i.e., doped, with a number of elements, asis well known in the art, but rare earth element stabilized zirconia,specifically yttria stabilized zirconia, is preferred as it hasexcellent oxygen ion mobility. A preferred composition is (ZrO₂)₀.09 (Y₂O₃)₀.10 as that material works well in solid oxide electrochemicalcells. Other mixed oxides can be used including yttrium doped thoriumoxide. The method of this invention is applicable to oxide layers whichtransfer oxygen in any form including monoatomic oxygen as well as ionicoxygen.

A preferred fuel electrode thickness is about 50 microns to about 200microns, though the thickness may be adjusted to the desired resistanceof the cell. An electronic conductor can be used in particulate form inthe fuel electrode 6. Metals are preferred as they are more conductiveand reduce cell resistance, but oxides can also be used. Metals arepreferred to metal oxides for a fuel cell because the atmosphere isgenerally reducing.

Metals that can be used as the electronic conductor particles 11 includeplatinum, gold, silver, copper, iron, nickel, cobalt and alloys andmixtures thereof. Metal oxides that can be used include chromic oxide,lanthanum chromite, and lanthanum manganite. The preferred materials arenickel, cobalt, alloys thereof and mixtures thereof, as these metals areless expensive, more stable, more sulfur resistant, and have anacceptable oxidation potential. The particles 11, which are preferablyfrom about 1 micron to about 5 microns diameter, may be applied tocontact the electrolyte as a powder layer in many different ways,including slurry dipping and spraying. Another method of application isa tape transfer technique, which is useful because of ease of massfabrication, registering of dimensions, and uniformity in thickness andporosity.

The material which binds the conductor particles to the electrolyte andprovides a skeleton partly embedding the conductor particles can beapplied by vapor deposition and formed from two reactants. The firstreactant can be water vapor, carbon dioxide or oxygen itself, and thesecond reactant can be a metal halide, preferably zirconiumtetrachloride, plus the halide of a stabilizing element, such as yttriumchloride. The skeletal binding material 12 is preferably selected to bethe same material as the electrolyte (or the same material modifed bydoping) so that a good bond forms between the binding material 12 andthe electrolyte 5 and there is a good thermal match between the twomaterials. Also, doping with, for example, transition metal elements,can provide a binding material which improves electrode performance. Thepreferred binding material is yttria stabilized zirconia although a widevariety of ceramic metal oxides that are compatible with the electrolytecan be used.

The skeleton structure 12, when deposited by vapor deposition, has beenfound to grow around the metal particles 11. In order to form theparticle embedded skeleton structure, a coating of the metal powderlayer is first applied to one surface of the solid oxide electrolyte.Then, an oxygen gas is applied to the other surface of the electrolytewhile a metal halide vapor is applied to the metal particle side. Theelectrolyte is heated to a temperature sufficient to induce oxygen todiffuse through the electrolyte and react with the halide vapor causinga skeletal coating to grow partly around the metal particles, asdescribed in greater detail in U.S. Pat. No. 4,597,170, hereinincorporated by reference.

Electrochemically active sites in solid state electrochemical cells arewhere the reactant, electrolyte and electrode interface. In the case ofFIG. 2, these electrochemically active sites 13 are where the fuel gas,F, is capable of combining with oxygen ions and where electron transferscan take place to generate an electric current. As can be seen, by usingan embedding skeleton alone, the number of active areas 13 is ratherlimited. Due to the fact that these electrodes have a relatively smallnumber of active sites, the cell can be rapidly affected by contaminantsthat can block these sites. Sulfur and sulfur species, for example, areabsorbed stronger on these sites than carbon monoxide and hydrogen andtherefore reduce overall cell performance.

FIG. 3 shows the electrode structure of this invention, where theelectrode 6 is bonded to a solid, oxygen ion-conducting electrolyte 5.The electrode comprises particles of electronic conductor 11 partlyembedded in a skeletal member of a ceramic metal oxide 12. The particlesand skeleton are covered, preferably completely, with anionic-electronic conductor coating 15. This coating layer can be denseor porous, depending on the technique of application. The coating can beapplied by any means, although simple impregnation from an aqueoussolution, a solvent solution, such as an alcohol solution, or a finesuspension, is preferred.

Useful ceramic oxides for the coating 15 are those that are both ionicand electronic conductive. Preferred ceramic oxides for the coating 15are doped or undoped ceria and doped or undoped urania uranium oxide. Bythe term "doped" is meant inclusion of from about 1 mole % to about 70mole % of a doping element, which either causes increased oxygen ionconduction or electronic conduction or both. High doping may result if amixture of dopants are used. Dopants for both ceria and uranium oxidecan include oxides of the rare earth metals, i.e. elements 57 to 71: La,Ce, Pr, Nd, Pm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, as well aszirconium, yttrium, scandium and thorium, and their mixtures. Thepreferred dopants are zirconia, thoria and lanthana.

The outer coating 15 is capable of free expansion and contraction duringfuel cell heat cycling. The coatings are thin and may be discontinuousas shown in FIG. 3. Coating cracking or partial flaking is notdeleterious to performance since most flaked particles would be trappedin the porous electrode structure and become active again in a newposition. The electrode active site triple point sites 13 are spreadover the surface of the outer coating 15, as shown in FIG. 3. Here, theionic-electronic conductive coating 15 is capable of transportingO.sup.═ ions, from the essentially only ionic conductive skeleton 12 tocontact fuel. Thus, the active sites are not limited to the juncture ofthe ionic conductive skeleton 12 and the metallic particle 11, but arevastly expanded because the coating layer conducts O.sup.═ ions,conducts electrons, and in most cases is porous to fuel gas. Thethickness of coating 15 is from about 0.5 micron to about 20 microns andis preferably continuously attached throughout the entire surface of theporous cermet electrode. Preferably, after application of the ceramicoxide coating 15, it is subjected to a heat treatment at temperaturesfrom about 500° C. to about 1400° C. to form the oxide coatings or todiffuse elements from the coating into the skeleton, introducingincreased electronic conduction into the skeleton, thus making theskeleton 12 more of an active part of the electrode.

In one method of coating, the metal particle embedded electrode isimprgnated with a metal salt solution, that, when heated up to operatingtemperatures of the solid oxide cells, i.e., over about 500° C.,decomposes or reacts to form the desired mixed oxide coating. Forexample, an aqueous mixed solution of lanthanum nitrate and ceriumnitrate can be used. When the solution dries and the salt is heated upit will decompose to a mixed oxide of lanthanum-doped cerium oxide. Thisoxide will coat the surface of the porous cermet electrode. This coatingis polycrystalline and is relatively weakly attached. It is protected,however, by the porous and rigid overall electrode structure.

As a result of this coating, the active electrode surface area isgreatly enlarged. Thermal cycling of electrode does not lead toelectrode deterioration, since the coating can freely expand. Anelectrode of this kind could also be prepared by impregnation ofelectrodes with fine oxide suspensions, which requires a very fineparticle size of the oxide. Other solution of salts can be usedadvantageously, for instance, a methanol solution of the hexahydrates ofcerium nitrate and lanthanum nitrate is preferred because of itssuperior wetting behavior.

While the electrodes of this invention are primarily useful in solidoxide fuel cells, they can also be used as electrodes for solid stateelectrolyzers and gas sensors.

EXAMPLE

A tubular cell closed at one end was prepared. It was 400 mm long and 13mm in diameter, consisting of a 2 mm thick porous support tube of calciastabilized zirconia, a 1 mm thick 40% porous air electrode of dopedlanthanum manganite on top of the support tube, and a 50 micron thickelectrolyte of yttria stabilized zirconia (ZrO₂)₀.90 (Y₂ O₃)₀.10 on theair electrode. A 100 micron thick layer of about five micron diameternickel powder was deposited over the electrolyte by means of slurrydipping. The nickel layer was about 50% porous. A ceramic skeleton ofyttria stabilized zirconia was deposited around the nickel powder layerto mechanically attach it to the electrolyte according to a processdescribed in U.S. Pat. No. 4,597,170, incorporated herein by reference.

The ceramic skeleton was about 1 to 5 microns thick and partly embeddedthe nickel powder layer extending throughout this layer from the surfaceof the electrolyte, but being thicker near the electrolyte. The finishedelectrode thus constituted a cermet consisting of nickel and reinforcingzirconia ceramic.

The porous cermet electrode was then impregnated with aroom-temperature, saturated solution of cerium nitrate and lanthanumnitrate, which was prepared by dissolving the hexahydrates of thesesalts in methanol. Methanol is the preferred solvent because of goodwetting characteristics but other solvents such as water and otheralcohols can also be used. The solution was applied by brushing. Theimpregnated electrode was then dried in a hood at room temperature.

The impregnated salts were thermally decomposed to a mixed oxide duringa heat-up procedure for testing the cell. The heating rate was about 1hour, from room temperature to 1000° C. The resulting impregnated oxidecovering was a lanthanum-doped cerium oxide (CeO₂)₀.8 (La₂ O₃)₀.2. Thetube had an active electrode area of about 110 cm², which wasimpregnated with 2.8 mg/cm² of the lanthanum doped cerium oxide solids.Microscopic examination of electrodes after testing showed that theoxide impregnation was embedded in the voids of the porousnickel-zirconia cermet electrode. The impregnated oxide weight varieswith electrode porosity and thickness and can be as low as 0.5 mg/cm² oras high as 10 mg/cm². Due to the fact that the impregnated finelydivided oxide covering was a mixed electronic-oxygen/ionic conductor, itis an integrated part of the electrode and provides additional activeelectrode area over that of an unimpregnated electrode. Therefore,higher current densities can be achieved whether the cell operates as afuel cell or as an electrolysis cell.

FIG. 4 of the drawings shows behavior of a cell at 900° C., beforeimpregnation (line A) and after impregnation (line B), and at 1000° C.,before impregnation (line C) and after impregnation (line D), when thecell operated as a fuel cell. Lines B and D correspond to the fuel cellconstruction of this invention and show very much improved results overlines A and C. The VI-characteristics were achieved with hydrogen/3% H₂O as fuel (less than 10% fuel utilization) and air. This test cell,utilizing the same cell geometry but a shorter section, was also testedin the electrolysis mode, and the performance is shown in FIG. 5, whichdemonstrates that the cell performance is also greatly improved when thecell operates as an electrolyzer of CO₂, H₂ O or mixtures of thesegases. FIG. 5 shows the IR-free overvoltage of the electrolyzer cellcharacteristic after 24 hours at 900° C. when operated withoutimpregnation (line E), and the improvement observed with impregnation(line F). The cathode gas consisted of a cell inlet gas mixture of 77%CO₂ and 23% H₂. The gas composition shifts according to the water gasreaction: CO₂ +H₂ →←H₂ O+CO. The cell, therefore, electrolyzes steam aswell as carbon dioxide. The greatly reduced electrode overvoltage ofline F attests to the uniqueness of this composite cermet electrode asan anode (in fuel cells) and as a cathode (in electrolyzers).

A major and unexpected result of this new electrode structure is thefact that it exhibits sulfur stability during operation. Thisimprovement is demonstrated in FIG. 6, where the cell voltage curve of afuel electrode of a solid oxide fuel cell made as described in thisExample, of 110 cm² active surface area impregnated with 2.8 mg/cm² oflanthanum-doped (ceria CeO₂)₀.8 (La₂ O₃)₀.2 is shown as line A. The celloperated at 1000° C. and at a constant fuel utilization of 85%, using afuel of 67% H₂, 22% CO and 11% H₂ O. The average current density was 250mA/cm². Fifty ppm of hydrogen sulfide was added to the fuel, which ledto a 4.7 percent performance loss. A similar size cell B of identicalconstruction but without the impregnation was exposed to the sameconditions, and an unacceptable performance loss was recorded in a veryshort time (line B), which increased continuously to an unacceptablelevel, while the electrode according to this invention stabilized aftera short time (line A).

I claim:
 1. A method of bonding a separate, porous, active layer on anelectronically conductive fuel cell electrode attached to a solidelectrolyte, oxygen ion transporting oxide layer comprising thesteps:(A) forming a coating of particles of an electronic conductorselected from the group consisting of nickel, cobalt, and mixturesthereof on a first surface of an oxygen ion transporting, solid oxideelectrolyte layer comprising zirconia; (B) applying a source of oxygento a second surface of said solid oxide electrolyte layer; (C) applyinga metal halide vapor to said first surface of said solid oxideelectrolyte layer; (D) heating said oxygen transporting, solid oxideelectrolyte layer to a temperature sufficient to induce oxygen todiffuse through said oxide layer and react with said metal halide vapor,whereby essentially only an ion conductive metal oxide skeletalstructure, comprising zirconia, grows partially around said particles ofsaid electronic conductor, embedding them, and forming an electrodeattached to the electrolyte; (E) coating both the particles and the ionconducting metal oxide skeletal structure with a separate, salt coatingwhich upon heating will form an oxide selected from the group consistingof ceria, doped ceria, uranium oxide, doped uranium oxide, and mixturesthereof; and then (F) heating the salt coated structure at a temperatureof from about 500° C. to about 1400° C., to form a separate, porous,ionic-electronic conductive, active coating comprising an oxide selectedfrom the group consisting of ceria, doped ceria, uranium oxide, dopeduranium oxide, and mixtures thereof, where the ionic-electronicconductive coating formed in step (F) is effective to provideelectrochemically active sites over its entire surface, and where theheating in step (F) is effective to diffuse elements from the saltcoating into the skeletal structure, introducing increased electronicconduction into the skeletal structure.
 2. The method of claim 1, wheresaid oxygen ion transporting oxide layer is an electrolyte tube and boththe electrolyte and metal oxide skeletal structure comprise stabilizedzirconia.
 3. The method of claim 1, where the ionic-electronicconductive coating formed in step (F) consists essentially of dopedceria, doped uranium oxide and mixtures thereof, where the dopant forceria and uranium oxide is selected from the group consisting ofzirconia and thoria, the halide vapor applied in step (C) compriseszirconia halide and where the ionic-electronic conductive coating formedin step (F) is effective to provide sulfur stability while operating inthe presence of sulfur species.
 4. The method of claim 1, where the saltcoating is one which upon heating will form an oxide selected from thegroup consisting of ceria, doped ceria, and mixtures thereof, which instep (F) will form an ionic-electronic conductive coating comprising anoxide selected from the group consisting of ceria, doped ceria, andmixtures thereof.